When the application runs for the first time, every things works fine. But when I press the Emulator HOME or BACK button and back to the application the problem appears : getHolder().lockCanvas(null) always returns null. What Android documentation says about this in SurfaceHolder's page is "A null is returned if the surface has not been created or otherwise can not be edited". When I back to the application the surface is still alive, I don't know what is happening.

Well, I have no more imagination to solve this for today (two days working hardly).
Can anyone help me?